1. Field of the Invention
The present invention relates to communication networks, and, more specifically, to mechanisms for automatically recovering from failures in mesh communication networks.
2. Description of the Related Art
A mesh communication network has a set of nodes interconnected by communication links. A path in a mesh network is a set of one or more links connecting a source node to a destination node possibly through one or more intermediate “transit” nodes.
In a wavelength-division multiplexing (WDM) optical mesh network, each link connecting a pair of nodes has one or more optical fibers, each of which can support transmission of optical signals at one or more different wavelengths. Using synchronous optical network (SONET) terminology, each wavelength in a link may be said to correspond to a SONET “line,” which is understood to correspond to a portion of the network delineated by a port at each end, each port being associated with line-terminating equipment at one of the two nodes connected by the link. Alternatively, using synchronous data hierarchy (SDH) terminology, each wavelength may correspond to an SDH multiplex section. In this specification, the terms “line” and “port” will be used to convey the concept of a SONET line or equivalently an SDH multiplex section. Note that, in a WDM network that also supports time-division multiplexing (TDM), such as a SONET-based network, each wavelength can itself carry a plurality of time-multiplexed demands, where a demand is the fundamental bandwidth unit of service supported on the network.
Mesh networks are preferably able to recover automatically from the failure of at least one node or link in the network. Such a network is considered to be a “protected” network. Recovery mechanisms for such networks can be either path-based or link-based. Path-based recovery is the process of recovering from a failure of one of the links or nodes in a path from a source node to a destination node by rerouting traffic for the entire path. In path-based recovery, the recovery path shares only the source and destination nodes with the original path. Link-based recovery, on the other hand, is the process of recovering from a single failure by rerouting traffic around the failure, without rerouting the entire path. In many instances, the recovery path for link-based recovery from the failure of a single link is identical to the original path with the exception of the failed link, which is typically replaced by two or more new links connecting one or more new nodes.
Path-based recovery mechanisms have both advantages and disadvantages relative to link-based recovery mechanisms. For example, path-based recovery mechanisms are often simpler to implement than link-based recovery mechanisms, because path-based recovery does not require knowledge of the specific failed link or node, only knowledge of the failure of the end-to-end path that includes the failed link or node. On the other hand, link-based recovery mechanisms are often faster than path-based recovery mechanisms, because link-based recovery typically involves the reconfiguration of fewer nodes.